Free the thread local context from an atexit callback

Free the thread local context from an atexit callback

They happen very late in the destruction process but that's not an issue
there. The MGWIZ structure and the SVs they contain are destroyed and
removed from the global pointer table when the wizard object is destroyed.
At the time the atexit callback fires, the pointer table is then empty and
the only thing left to do is to free its own (shared) memory.